Dual power brake booster

ABSTRACT

A vehicle brake booster and master cylinder assembly has a vacuum suspended booster section which is controlled by movement of the vehicle brake pedal. A hydraulic booster section is in series with the vacuum suspended booster section, and a master cylinder unit is in series with the hydraulic booster section. The assembly is so arranged that in normal operation the vehicle operator obtains booster brake actuating pressures by operation of the vacuum suspended booster which acts through the hydraulic booster mechanism without operating the hydraulic booster. When greater brake actuating pressures are required, as indicated by increased brake pedal force exerted by the operator, the vacuum booster reaches its limit or run-out condition and the hydraulic booster is operated so as to continue the increase in master cylinder output pressure. If still further master cylinder output pressure is required when the hydraulic booster has reached its run-out condition, the master cylinder is actuated manually through mechanical force transmitting elements which are parts of the booster sections. The vacuum suspended booster section is connected to the vehicle engine intake manifold as a vacuum source, and the hydraulic booster section is connected in the hydraulic power steering gear system so that hydraulic pressure for its operation is generated by the power steering gear pump. The assembly is so interconnected that the master cylinder can be operated manually when there is no or insufficient power pressure to operate either booster section. It may also be operated by actuating the vacuum suspended booster section, and, when no hydraulic pressure is available to the hydraulic booster section, this operation may be followed by manual actuation. It may be operated by initially actuating the hydraulic booster section when there is no vacuum available for initially operating the vacuum suspended booster section, followed by manual actuation as needed. In one embodiment the assembly utilizes a single hydraulic fluid for the hydraulic booster section and for the master cylinder and brake apply circuitry. In another embodiment the hydraulic booster section is operated by a separate fluid circuit which is fluidly independent of the master cylinder and brake apply pressure circuitry.

This is a division of U.S. application Ser. No. 441,239, filed Feb. 11,1974.

The invention relates to a dual source power brake booster assembly, andmore particularly to an assembly having in series a vacuum suspendedbooster section and a hydraulic booster section connected to the mastercylinder assembly. It is a feature of the invention to provide forcetransmitting paths and valve operating mechanisms interconnecting thetwo power booster sections so that the sections may be operatedsequentially when power is available to both sections, followed bymanual operation if necessary, or either section may be operated whenthe other section has no power available, followed by manual operationif necessary.

In more particular aspects of the invention, the connection between thebooster sections is provided by two members acting as output members ofthe vacuum suspended booster section, one member being a sleeve and theother member being a plunger reciprocably received through the sleeve.The sleeve is positioned to operatively engage the hydraulic boosterpiston and therefore act as one input to the hydraulic booster section,while the plunger is fitted with a valve element for controllingoperation of the hydraulic booster section in cooperation with anothervalve element mounted on the hydraulic booster section power piston. Thesleeve and plunger are so arranged that during power operation of thevacuum suspended booster section they move as a unitary member.Therefore the boosted force from the vacuum suspended booster istransmitted through the sleeve to the hydraulic power pistonmechanically to the master cylinder unit. Concurrently, the plungermoves with the sleeve and therefore the plunger-connected valve elementof the hydraulic booster section moves with the hydraulic power pistonconnected piston valve element so that no valving action takes place andthe hydraulic booster section is not power operated. When the vacuumsuspended booster section reaches power run-out, which can occur at anypoint in its operation depending upon the amount of vacuum available,further movement of the input push rod of the vacuum booster section bythe vehicle operator establishes a mechanical force transmitting driveconnection from the input push rod through the input portion of thevacuum suspended booster section valve to the output plunger so that theoutput plunger is moved toward the hydraulic booster section relative tothe sleeve. This causes the valve elements of the hydraulic boostersection to actuate the hydraulic booster section and the power pistonthereof is moved in a master cylinder actuating direction underhydraulic power. Thus additional master cylinder output pressure isgenerated under hydraulic power.

When the hydraulic power booster section reaches power run-out, whichfor the purposes of this disclosure and the appended claims encompassesany power supply condition from complete absence of hydraulic powerpressure availability to full actuation of the hydraulic booster sectionby the maximum hydraulic power normally available, the plunger will movesufficiently to establish a mechanical force transmitting driveconnection so that manual force exerted by the vehicle operator on thebrake pedal is mechanically transmitted to the master cylinder unit.

In one embodiment of the invention a single hydraulic fluid is used forthe hydraulic power section and the brake master cylinder section. Inthis arrangement the master cylinder reservoir acts as the reservoir forthe power steering and hydraulic booster circuitry as well as the brakeactuating pressure circuitry. In another embodiment of the invention themaster cylinder is separated from the hydraulic booster section and eachof these sections is provided with its own hydraulic circuit. In eitherembodiment the hydraulic booster section is provided with a pressurerelief valve which limits the amount of booster power pressure availableby opening at a predetermined power pressure to bypass the booster powerchamber and thereby prevent undue pressure build-up in the hydraulicbooster.

IN THE DRAWINGS

FIG. 1 is a schematic representation of a brake system embodying theinvention.

FIG. 2 is a graph indicating the performance of the brake booster undervarious operating conditions.

FIG. 3 is a cross section view of a brake booster and master cylinderassembly embodying the invention.

FIG. 4 is a modification of the assembly of FIG. 3, showing the modifiedassembly in cross section with parts broken away.

The vehicle 10, schematically represented by vehicle portions such asthe firewall 12 and a support member 14, is provided with a brakebooster and master cylinder assembly 16 embodying the invention. Certainother portions of the vehicle are also schematically illustrated andinclude the vehicle front wheel brakes 18, rear wheel brakes 20, a pump22, power steering gear 24, and suitable conduits. These conduitsinclude conduit 26, which interconnects pump 22 and the power steeringgear 24 to conduct hydraulic fluid from the pump output port 28 to thepower steering gear input port 30. Conduit 32 connects the powersteering gear output port 34 with the hydraulic fluid input port 36 ofassembly 16. The hydraulic fluid output port 38 of assembly 16 isconnected by conduit 40 to the pump 22. In the system schematicallyillustrated, pump 22 has a fluid reservoir section 42 into which conduit40 delivers hydraulic fluid. When the modified booster of FIG. 4 isused, pump 22 need not be provided with a reservoir 42 since the mastercylinder reservoir acts as the hydraulic fluid system reservoir andconduit 40 is connected between that reservoir and the pump. Thisarrangement is described below in further detail.

The assembly 16 includes a differential air pressure operated boostersection 44, a hydraulic fluid pressure operated section 46, and a mastercylinder section 48. The master cylinder section has brake actuatingpressure conduits 50 and 52 respectively connecting the pressurizingchambers of the master cylinder with the front wheel brakes 18 and therear wheel brakes 20. A brake pedal assembly 54 is pivotally mounted onthe vehicle support member 14 at pivot 56 and is also connected to pushrod 58 to move the push rod generally axially to control the assembly 16and transmit manual force thereto. The booster section 44 is illustratedas being of the vacuum suspended type and is connected by check valve 60and conduit 62 to a suitable source of vacuum such as the vehicle engineintake manifold 64.

FIG. 3 shows the assembly 16 in cross section and in greater detail. Thebooster section 44, which may be considered to be the first boostersection since it is the first section to be operated, is a vacuumsuspended brake booster of the general type disclosed in U.S. Pat. No.3,249,021, entitled "Power Brake Booster." It is essentially a singlediaphragm, vacuum suspended booster having a lever reaction system. Ithas a housing 66 which has a movable power wall 68 received therein anddividing the housing into a substantially constant pressure chamber 70and a variable pressure chamber 72. Vacuum is maintained in chamber 70by check valve 60 and the usual operation of the vehicle engine to whichthe booster is connected by conduit 62. The power wall 68 includes adiaphragm 74, a diaphragm support 76, and a piston 78 to which thediaphragm and its support are secured. Piston 78 has a rearwardextension 80 which is slidable through a seal and bearing 82 provided inthe rear section 84 of housing 66. A reaction retainer 86 is removablysecured to piston 78. The push rod 58 has a ball end 88 received in apocket of the air valve 90. Valve 90 is the input member of the valvemechanism which controls this booster section. Ball 88 is staked inplace to provide a pivotal connection between rod 58 and air valve 90but otherwise effectively joining them together as an input member. Theair valve 90 is adapted for sliding movement through a portion of piston78 and is arranged to meter the flow of atmospheric air to chamber 72. Afloating valve 92 engages the inside wall of extension 80 of piston 78and is held in its operative position by a retainer 94. Another portionof the floating valve 92 is maintained in engagement with the air valve90 when the booster section is in the released position by a spring 96and cup 98. A vacuum valve 100 is formed as a part of power piston 78and when the booster section is in the released position floating valve92 is slightly spaced from this valve so that vacuum is communicatedfrom chamber 70 to chamber 72 through appropriate passages.

As is well known in the operation of this type of booster controlmechanism, slight leftward movement of push rod 58 and air valve 90, asviewed in FIG. 3, permits floating valve 92 to engage vacuum valve 100to close the vacuum connection between chambers 70 and 72. Furthermovement of the air valve 90 causes the air valve to open relative tofloating valve 92 and meter air at atmospheric pressure into chamber 72.This creates a pressure differential across power wall 68, moving thepower wall leftwardly against the force of the power wall return spring102. This also moves vacuum valve 100 and floating valve 92 leftwardlyuntil the floating valve reengages air valve 90. The booster is then inthe poised position, holding this position since both the air valve andthe vacuum valve are closed. Release of the brake pedal by the operatorallows push rod 58 and air valve 90 to move rightwardly, liftingfloating valve 92 off of vacuum valve 100, thereby reestablishing thevacuum connection between chambers 70 and 72 decreasing the differentialpressure across power wall 68 so that power wall return spring 102 movesthe power wall back to the release position shown in the drawing.

The lever reaction mechanism of booster section 44 includes a leverreaction plate 104, reaction levers 106 disposed against one edge of thelever reaction plate, and a shoulder 108 on piston 78. The reactionmechanism further includes an air valve spring 110, one end of whichengages the inner ends of lever 106 and the other end of which is seatedon spring retainer 112, which is fastened to air valve 90. The forwardface 114 of air valve 90 has a snubber 116 which engages the inner endsof levers 106 and then yields to permit engagement of these lever innerends with air valve face 114 when the booster is actuated. Thisarrangement provides a sense of feel to the vehicle operator duringoperation of the power booster, as is well known in the brake boosterart.

The point at which booster section 44 differs from U.S. Pat. No.3,249,021 in any material extent is in the arrangement of the boostersection output mechanism. This mechanism includes a first output formedas a sleeve 118. The rear end 120 of sleeve 118 is slidably received inan aperture 122 of reaction retainer 86, the aperture and sleeve beingaxially aligned with air valve 90. Sleeve end 120 abuts lever reactionplate 104, and the sleeve forward end 124 extends through the forwardsection 126 of the booster housing 66 in sliding and sealed relation. Aplunger 128 provides the second output of booster section 44 and isslidably received in sleeve 118 and is sealed in sliding relation withthe inner wall of that sleeve by seal 130. The rear end of plunger 128has a shoulder 132 abutting lever reaction plate 104. A reduced diameterextension 134 of plunger 128 extends rearwardly from shoulder 132through an aperture 136 centrally formed in lever reaction plate 104.The rear face 138 of extension 134 is in alignment with face 114 of theair valve 90 so that sufficient leftward movement of the air valve cancause mechanical engagement of the air valve and the plunger extensionin force transmitting relation. The space between faces 114 and 138,with the booster section in the released position illustrated, issufficient to permit normal valve controlling operation of the boostersection so long as there is sufficient power available to operate thebooster section. The forward end 140 of plunger 128 extends through andbeyond the forward end 124 of sleeve 118 and is formed to provide avalve element of the valve controlling the hydraulic booster section 46as will be described below.

When the brake booster section 44 is operated by movement of the inputmember formed by push rod 58 and air valve 90, force is transmitted fromthe power wall 68 through the reaction levers 106 and reaction plate 104to sleeve 118 and plunger 128. The inner ends of reaction levers engagethe air valve snubber 116 and face 114 to transmit a small amount ofreaction force back to the brake pedal so that the operator can sensethe amount of brake booster force being generated. The major portion ofthe reaction force is transmitted to power piston 78 through reactionlevers 106 and shoulder 108. The sleeve 118 and plunger 128 will movetogether due to their arrangement in abutting plate 104, this movementbeing leftward in the brake applying direction. If the booster section44 is actuated to a power run-out condition, further movement of airvalve 90 leftwardly by the manual force exerted on the brake pedal willcause the air valve to engage plunger extension 134 and move the plungerleftwardly relative to sleeve 118. The extension 134 slides in aperture136 of plate 104, separating shoulder 132 from engagement with theplate. Thus in normal brake booster section actuation both output 118and 128 move in a unitary manner. The relative movement of theseoutputs, in which plunger 128 moves leftwardly relative to sleeve 118,will occur at any time that the operator exerts sufficient manual forcedemanding additional output force which the booster section 44 is unableto generate. Thus after vacuum connection 62 is broken, or there is noadditional vacuum available from the vacuum source, and the boosterchamber 72 is at atmospheric pressure, leftward movement of air valve 90can not cause further power actuation of the booster section. Air valve90 will therefore actuate plunger 128 by transmitting the manual forcemechanically thereto as above described. For the purpose of thisdisclosure and the appended claims, any condition of either boostersection in which the booster output force is not sufficient to meet thedemand force is referred to as booster power run-out. The term thereforeincludes a total lack of power for operation of a booster section, apartial availability of such power, or conditions wherein the fullyavailable power is present and has been used.

The hydraulic booster section 46 includes a housing 146 having a bore148 therein, the rear end of the bore having a rear wall 150 throughwhich an opening 152 is provided. A power piston 154 is reciprocablyreceived in bore 148 and provides a power wall for the booster section.An abutment 156 on the rearward side of piston 154 is in engagement withthe forward end 124 of sleeve 118, this sleeve forward end extendingthrough opening 152 in sealing relation therewith. Plunger end 140extends through an opening provided in abutment 156. The valve element158 is mounted on piston 154 and cooperates with valve element 160 onthe forward end of plunger 128 to provide the control valve for thehydraulic booster section 46. This valve is of the open center type andtherefore the valve elements are spaced sufficiently apart axially toprovide substantially unrestricted flow of hydraulic fluid through thevalve so long as the hydraulic booster section is not actuated. Piston154 has a passage 162 on the low pressure side of valve element 158which communicates the side of the valve with the exhaust chamber 164.This chamber is on the forward side of piston 154 and is formed by theforward portion of bore 148. The booster section power chamber 166 is onthe rearward side of piston 154 and upstream of the valve formed byelements 158 and 160. Piston 154 has a push rod-like extension 168 whichextends through chamber 164 and the end seal 170 defining the forwardend of that chamber. Extension 168 forms the output member of thehydraulic booster section. It also extends through the end seal supportand retainer 172, which has a generally dished annular configuration andis secured in the forward end of housing 146. The housing forward endand this retainer are constructed to provide for a drain and vent whichwill provide a path for removal of any hydraulic booster fluid which mayleak through seal 170 to prevent its possible contamination of mastercylinder brake fluid.

Port 36 is connected by passage 174 to power chamber 166. Port 38 isconnected to exhaust chamber 164. A pressure relief or bypass valvepassage 176 connects passage 174 and chamber 164. Pressure relief valve178 is mounted in passage 176. The valve is normally closed but isopened when pressure in inlet passage 174 exceeds the predeterminedpressure required to open the valve. When the valve opens, it relievesthe excess input pressure through passage 176 and exhaust chamber 164 toport 38, conduit 40 and reservoir 42.

The hydraulic booster section is in the position shown in FIG. 3 whenreleased, piston return spring 180 holding the piston abutment 156against the forward end of sleeve 118. When the vacuum booster section44 is power operated, sleeve 118 moves piston 154 leftwardly againstspring 180, therefore moving the extension 168 in the brake mastercylinder actuating direction. Since lever reaction plate 104 movesplunger 128 concurrently as a unit with sleeve 118 in this condition ofoperation, the valve elements 158 and 160 do not change in theirrelationship and therefore hydraulic fluid continues to be circulatedthrough the hydraulic booster section without generating a powerpressure in chamber 166, and the boosted force exerted through sleeve118 is mechanically transmitted through abutment 156, piston 154 andextension 168. When, for any reason as above described, plunger 128moves leftwardly relative to sleeve 118, valve element 160 moves towardvalve element 158 to restrict flow therethrough and cause a build-up ofpressure in power chamber 166. This pressure acts on piston 154 to movethe piston leftwardly, the extension or output member 168 thereforemoving in the master cylinder actuating direction because of the addedforce generated by the hydraulic power booster section. The valveelements may reach a poised position wherein the amount of restrictionto flow therethrough creates just enough pressure in power chamber 166to balance power piston 154 against spring 180 and the reaction forcefrom the master cylinder section exerted on extension 168. An annulareffective reaction area on plunger 128 exposed to pressure in powerchamber 166 will provide a proportional feel to the vehicle operator. Itis noted that a T-shaped vent passage 182 in the forward end of plunger128 vents an area between seal 130 and a similar seal 184 to exhaustchamber 164 so that any hydraulic fluid which might leak past seal 184will be returned to the hydraulic booster section and will not enter thevacuum booster section. A suitable vent 186 is provided between walls150 and 126 to prevent the entry of any hydraulic fluid into the vacuumbooster section which passes the seal 188 between wall 150 and sleeve118.

The master cylinder section 48 is of the tandem type in which primaryand secondary pistons are received in a common bore and pressurize brakefluid in separate pressurizing chambers for separate brake actuatingcircuits. The section includes a housing 190 in which bore 192 isformed. Primary pressurizing piston 194 and secondary pressurizingpiston 196 are reciprocably received in bore 192 and respectively definetherewith primary pressurizing chamber 198 and secondary pressurizingchamber 200. Piston return spring 202 in chamber 200 urges piston 196against the caged spring retainer 204. This retainer is slidably mountedon headed bolt 206, which is secured to the forward end of primarypiston 194. Spring 208 is caged between piston 194 and retainer 204.When the assembly is in the brake released position illustrated thisprovides a precise positioning of the secondary piston 196. Piston 194is precisely positioned against its stop 210 and the caging mechanismprecisely positions piston 196 relative to piston 194. Suitable cupseals 212 are provided on the pistons, as is well known in the art.

The master cylinder reservoir 214 includes a single primary reservoirsection 216 and secondary reservoir sections 218 and 220. Each of thesecondary reservoir sections has compensation ports 222, 224 and 226,228 which provide for compensation of the brake circuits connected toeach of the pressurizing chambers. The reservoir construction is basedon the disclosure of U.S. Pat. application Ser. No. 408,461, entitled"Master Cylinder" filed Oct. 23, 1973, which is a continuation of U.S.application Ser. No. 379,349, filed July 16, 1973, now abandoned. Theparticular construction is claimed in copending U.S. Pat. applicationsSer. NO. 462,335, now U.S. Pat. No. 3,877,228 and Ser. No. 462,355, nowU.S. Pat. No. 3,886,747, each entitled "Master Cylinder Assembly andReservoir For Same" and filed Apr. 19, 1974. A baffle 230 is providedfor secondary reservoir 218 and a dam 232 is provided for secondaryreservoir 220 to increase the capacity of that secondary reservoir. Asuitable fluid level sensor 234 is provided in the primary reservoirsection 216.

Primary pressurizing chamber 198 is connected by port 236 to brakeconduit 52 and secondary pressurizing chamber 200 is connected by port238 to brake conduit 50. While the schematic illustration of FIG. 1shows the secondary pressurizing chamber 200 connected to the frontbrake circuit conduit 50 and the primary pressurizing chamber 198connected to the rear brake circuit conduit 52, in some installationsthis may be reversed so that the front brakes are actuated by brakepressure generated in the primary pressurizing chamber 198.

The rear face of piston 194 has a socket 240 which receives the end 241of extension 168 so that the output member formed by extension 168 is indirect force transmitting and drive relation with piston 194. Whenoutput member 168 is moved leftwardly under influence of any force orcombination of forces as described above, piston 194 moves leftwardly sothat its cup seal 212 closes compensation port 226 and it begins topressurize fluid in 198. The pressurized fluid and the force exertedthrough spring 208 combine to move secondary piston 196 leftwardly sothat its cup seal 212 closes compensation port 222, the pistoncontinuing to move against the force of return spring 202 to pressurizebrake fluid in chamber 200. The brake actuating pressures so generatedin chambers 198 and 200 are respectively delivered to the rear brakes 20and the front brakes 18 to actuate the vehicle wheel brakes.

It can be seen that the master cylinder can be actuated to generatebrake actuating pressure by any of several methods. When the vehicleoperator exerts manual force on the brake pedal it is transmittedthrough push rod 58 to air valve 90 and the vacuum booster section 44.The booster force is transmitted through sleeve 118, abutment 156,piston 154, and extension 168 to move master cylinder piston 194. As isshown in FIG. 2 of the drawing, in normal brake operation where brakepower is available to both booster sections, the master cylinderpressure generated in chambers 198 and 200 will follow along curve 242with an increase in brake pedal force. The vacuum booster will reach itsrun-out point at 244 on curve 242. The hydraulic booster section willthen become operative if the vehicle operator demands more mastercylinder pressure than that generated at this point of operation. Thisis accomplished by moving plunger 128 leftwardly relative to sleeve 118by mechanical engagement of the air valve 90 with the plunger,restricting hydraulic fluid flow between valve elements 158 and 160 tobuild up pressure in power chamber 166 and continue actuation of themaster cylinder along curve 242 by hydraulic brake boost operation. Thehydraulic boost force added to the manual and vacuum boost force willincrease with additional pedal force so that the master cylinderpressure may reach point 246. At this point, power run-out of thehydraulic booster section occurs and additional pedal force causes amanual additional actuation of the master cylinder section so that themaster cylinder pressure follows the portion of curve 242 extending pastpoint 246 at a lesser slope.

Curve 248 of FIG. 2 illustrates the operation of the entire assemblywhen there is no power pressure available for either booster section andthe master cylinder is actuated totally by manual force transmittedmechanically through both booster sections to the master cylindersection 194. Curve 250 of FIG. 2 illustrates the operation of theassembly when vacuum booster section 44 has been actuated along curve242 to point 244 but no pressure is available to operate the hydraulicbooster section 46 and therefore additional force must be appliedmanually. Point 252 may be reached when the booster has been operatedwith no power to the vacuum booster section 44. Such operation willcause an immediate operation of the hydraulic booster section 46 whensufficient movement of the air valve 90 has occurred to mechanicallyengage plunger 128 and additional pedal force is then applied. Themaster cylinder pressure will increase with pedal force along curve 242to point 252, which represents hydraulic pressure run-out. Additionalmaster cylinder pressure increase follows along curve 254 as additionalpedal force is exerted manually through the booster sections to themaster cylinder.

The modification shown in FIG. 4 has many parts which are the same asthe parts in the structure shown in FIG. 3 and the same referencenumerals and descriptions therefore apply. The vacuum booster section 44is constructed in the same manner and operates as described above. Themodified hydraulic booster section 260 operates in the same manner asdoes hydraulic booster section 46 and the master cylinder section 262operates to pressurize brake fluid in the same manner as does the mastercylinder section 48 of FIG. 3. The difference in construction andarrangement provide for the use of a single hydraulic fluid in themaster cylinder section and the hydraulic booster section, with themaster cylinder reservoir 214 acting as the hydraulic fluid reservoirfor the hydraulic system including the power steering pump, powersteering gear, and front and rear wheel brake circuits. To accomplishthis, the power piston 194 has its extension modified so that theextension is also the master cylinder primary piston, or may be directlyconnected thereto. Thus the hydraulic brake booster bore 264 is providedin a common housing 266 with the master cylinder bore 268. The boresintersect shortly to the rear of compensation port 226 at a slopingshoulder, bore 264 being the larger bore. The piston head 270 of powerpiston extension 272 is reciprocably received in master cylinder bore268. Exhaust chamber 274 of the hydraulic booster section is locatedbetween power piston 194 and piston head 270. Compensation port 276communicates chamber 274 with the secondary reservoir 220 and through itwith the primary reservoir chamber 216 of reservoir 214. By thisarrangement hydraulic fluid in reservoir 214 is at all times in fluidcommunication with the hydraulic brake booster exhaust chamber 274 andalso with pump 22 through conduit 40. In this arrangement conduit 40 isconnected to reservoir 216 through port 278 in the reservoir wall ratherthan through port 38 as illustrated in FIGS. 1 and 3.

The invention provides a dual power brake with numerous advantages overpresent brake systems. In normal operation it delivers the highperformance level obtainable from a hydraulic brake booster, andoccupies less space than would a vacuum brake booster of equivalentperformance. The two power sources are completely independent of eachother so that the assembly will operate if power is provided by eitherpower source. The assembly provides for effective manual operation whenno power is available from either or both power sources. It retains theadvantageous features of current vacuum powered boosters by havingsufficient vacuum stored to provide several power braking stops when thevehicle engine is not running. It lends itself to a basic plumbingcircuit requiring only the hydraulic connections to and from the brakebooster section when the structure of FIG. 3 is used, or only to thebrake booster section and from the master cylinder section reservoirwhen the construction of FIG. 4 is used. It provides the same brakepedal feel to the vehicle operator that is provided in currentproduction vehicles using vacuum suspended power brake boosters. Sinceit operates initially with the vacuum booster section to a braking levelencompassing the requirements for about eighty percent of the vehiclestops, most vehicle stops may be accomplished without requiringoperation of the hydraulic booster section. This substantially reducesthe power steering pump duty cycle as compared to hydraulic brakebooster systems which require the pump to generate pressure each timethe brakes are applied. The system will retain power brake availabilitywhen the power steering pump does not run, when the power steeringhydraulic circuit loses pressure for any other reason, or when there isa loss of vacuum power for the vacuum booster section. At all times itprovides a direct manual follow-through brake actuation upon which thevehicle operator can ultimately rely even if he loses power to bothpower booster sections.

In some installations it is desirable to utilize different reactionratios in the two booster sections. For example, the reaction ratio inbooster section 44 may be such that the operator receives a relativelysmall amount of the reaction force to provide feel as compared to thatin booster section 46. This would permit the operator to increase thebrake actuating pressure to some desired level with a certain desiredpedal force, but would require a higher rate of increase in pedal forceto further increase brake actuating pressure. This tends to provide alimiting action which would discourage the vehicle operator fromgenerating excess brake actuating pressures during high decelerationstops. In some other installations it may be desirable to utilize arelatively high reaction ratio in booster section 46 and a relativelylow reaction ratio in booster section 44. This would result in thegeneration of brake actuating pressure to a first value with arelatively high pedal force in relation to the brake actuating pressure,and would then permit further brake actuating pressure increase with asmaller increase in brake pedal force. Such an arrangement may beadvantageous on roads having widely varying tire-to-road frictioncharacteristics. For example, if a vehicle having such an arrangement isbeing braked on roads that have ice or snow on them, and also at timesdry, this would require a relatively large increase in the brake pedalforces initially applied to obtain comparable increases in brakingactuating pressures, thereby giving the vehicle operator an opportunityto modulate the brake actuating pressure precisely. When braking on dry,high friction characteristic roads, an increase in brake pedal forcewhich would cause booster section 44 to reach its run-out limit and thencause actuation of booster section 46 would give a higher rate ofincrease in brake actuating pressure for a given increase in brake pedalforce during actuation of booster section 46. This would give a desiredhigh braking force characteristic when close modulation of the brakeactuating pressure would not be necessary.

When the assembly 16 operates in the normal manner with booster section44 being actuated to its limit, followed by actuation of booster section46, the reaction ratios obtained are established by the ratio of thecontact points on levers 106 for the booster section 44, and thedifference in area of plunger 128 and the effective area of valveelements 160 and 162, as exposed to pressure in chamber 166, for boostersection 46. In installations where the booster ratios are substantiallythe same, the rate of increase in brake actuating pressure in relationto the amount of manual force applied through push rod 58 will besubstantially the same for each booster section. When the assembly isoperated without any vacuum available for power operation of boostersection 44, however, the reaction ratio of booster section 46 ismodified by the increase in reaction area represented by the area fosleeve end 124 exposed to hydraulic pressure in power chamber 166. Thistherefore changes the reaction ratio during operation of booster section46 so that the rate of brake actuating pressure increase is less for agiven amount of increase in brake pedal force than is the case whenbooster booster sections are fully operable. It is advantageous tomodify the reaction ratio in the hydraulic booster when it is theinitially actuated booster so as to improve modulation at low brakeactuating pressure levels. The vehicle operator therefore has a greaterrange of pedal force which may be exerted to obtain a given range ofbrake actuating pressure than he would have if booster section 46 wereactuated with the same reaction ratio under all conditions of operation.

What is claimed is:
 1. A brake booster and master cylinder assemblycomprising:a differential air pressure operated first booster sectionhaving first and second output members movable as a unit under power andrelatively movable under manual operation; a hydraulic pressure operatedsecond booster section in alignment with said first booster section andhaving a power piston and open center valve control means including afirst valve element on said piston and a second valve element cooperablewith said first valve element to restrict hydraulic flow therebetweenupon movement of said second valve element toward said piston and saidfirst valve element to actuate said second booster section; said firstbooster section first output member operatively engaging said secondbooster section power piston in force transmitting relation to move saidpower piston and therefore said first valve element commensuratelytherewith, said first booster section second output member beingoperatively connected with said second valve element in forcetransmitting relation to move said second valve element commensuratelytherewith; and master cylinder brake actuating pressure generating meansactuated by movement of said power piston; said first and second valveelements moving as a unit so long as said first booster section firstand second output members move as a unit so that said second boostersection is not power actuated, said second valve element moving towardsaid first valve element when said first booster section second outputmember moves relative to said first booster section first output memberto power actuate said second booster section, said second valve elementoperatively engaging said second booster section power piston uponsufficient movement relative to said first valve element to provide amechanical force transmitting path through said second booster sectionfor manual operation of said master cylinder brake actuating pressuregenerating means; said assembly being operable to generate brakeactuating pressure by sequential first booster section power operationand second booster section power operation and manual operation, and byany of said operations.
 2. A brake booster assembly for power actuationby first and second independent power sources and comprising:a firstpower section for power actuation by the first power source and having afirst input and first and second outputs and a first movable power wallconnected to move said outputs as a unit under power in response tomovement of said first input, and means for moving said second outputrelative to said first output by movement of said first input when saidfirst movable power wall has insufficient power to move; a second powersection for power actuation by the second power source and having secondand third inputs, a second movable power wall, a third output movable bysaid second movable power wall, and valve control means including avalve for control of the power actuation of said second power section,said valve having one valve element on said second movable power walland moving therewith and another valve element on said second input forcontrolling the application of power to said second movable power wall;said second output being connected to move said second input and saidfirst output being connected to move said third input whereby saidsecond power section valve is actuated by relative movement between saidvalve elements only when said second output and said second input aremoved by said first input relative to said first output and said thirdoutput.
 3. A brake booster assembly having an input member and an outputmember;a first power section having said input member as the inputthereof; a second power section having said output member as the outputthereof; said first power section having two parallel-acting outputswhich are also inputs for said second power section; first control meansfor said first power section including first valve means controlling theactuation of said first power section in accordance with movement ofsaid input member and subject to power provided thereto, said firstcontrol means having means mechanically transmitting force from saidinput member to one of said outputs and inputs upon run-out of power insaid first power section to move said one output and input relative tothe other of said outputs and inputs; second control means for saidsecond power section including second valve means controlling the poweractuation of said second power section in accordance with the movementof said second power section one input relative to said second powersection other input as aforesaid and subject to power provided thereto,said second power section one input being operatively mechanicallyengageable with said output member in force transmitting relation toprovide a mechanical drive path through said power sections from saidinput member to said output member upon run-out of power in said secondpower section.
 4. In a brake booster having a power wall; valve meansselectively controlling the pressure differential across said power wallto control said booster, said valve means including an input member;output mechanism slidably connected to said power wall; and reactionmeans connecting said output mechanism and said power wall and saidinput member in force transmitting relation whereby in power operation aminor portion of the total reaction force from said output mechanism isreceived by said input member and a major portion of the total reactionforce from said output mechanism is received by said power wall; theimprovement comprising:said output mechanism including a sleeve havingone end slidable in said power wall and abutting said reaction means,and a plunger slidably received in said sleeve and having one end formedwith a shoulder abutting said reaction means and a reduced sectionextending through said reaction means in abuttable force transmissiblealignment with said valve means input member, the other ends of saidsleeve and said plunger being outputs of said output mechanism; saidsleeve and said plunger moving axially as a unit during powered movementof said power wall, said valve means input member engaging said plungerupon run-out of power and mechanically moving said plunger axiallyrelative to said reaction means and said power wall and said sleeve uponfurther movement of said valve means input member.